It is previously known when certain process liquids are filtered to filter particles which are smaller than the filter meshes of a filter element by first building up a layer of particles on the filter surface which are larger than the filter meshes and in this way creating a three-dimensional deep filter layer of particles on the filter surface which itself functions as a filter for smaller particles which would otherwise pass through the filter element. Such “coating filters” are therefore suitable for liquid suspensions containing very small particles which can be caught therein.
Conventional coating filters can be built up on a filter element and consist of a suspension containing an aid addition of, for example, cellulose powder to the process liquid to be purified by filtering. This suspension is filtered through the filter element first in order to build up a coating layer on it, after which the process liquid itself can be filtered through the coating. When the coating layer has become saturated (clogged) with particles and the filtrate flow has decreased to too low a level, filtering is interrupted and the filter is cleaned. This leads to a marked reduction in production. This method also requires consumption of filter aid and gives rise to a large quantity of costly waste, which results in heavy system investments and high operating costs.
SE 514 311 C2 describes a method of the kind referred to in the introduction, in which a coating layer of particles originating from the process liquid itself is built up on the outside of a filter element on a perforated drum. A number of filtering units of this kind can work in parallel in a common tank in order to bring about high filtering production, that is to say a large filtrate flow. When the filtrate flow from the filtering units has decreased to a minimum value, which has resulted in a rise in level of the suspension in the tank which is sensed by a liquid level monitor common to the filtering units, filtering is interrupted in all the filtering units for the purpose of cleaning the filter elements by slow rotation of the drums against a scraping device on the outside of the filter elements or simply by rapid rotation of the drum. The result is an intermittent operating procedure and thus a reduction in production when the filter units are to be cleaned.